1. Field of the Invention
This invention relates to a method of analog signal processing and, more particularly, to an improved method of separating noise from information in a signal.
2. Description of Prior Art
When information is communicated through the process of transmission and reception, the received information signal contains in addition to the original information portion an unwanted signal portion due to noise, which degrades the original signal. When the noise falls within the spectrum or bandwidth of the information signal, it is often difficult to distinguish noise from signal.
This noise problem is particularly vexing in FM reception in moving vehicles. In the case of FM transmission, FM modulated radio waves carrying a non-repetitive signal of voice or music are transmitted from a fixed location. The radio waves arrive to a receiving location both via a direct path and via many different reflected paths. Since a receiving antenna output is an electric field which is the composite of the electric fields generated by the radio waves arriving both directly and after reflection, radio waves from different paths, produce constructive and destructive interference resulting in what is commonly referred to as "multipath" noise. In the case of a fixed receiver, a directional antenna can be used to favor reception of a radio signal from a preferred path and to reject signals from other paths; alternatively, the antenna may be moved to a near by location where signals from different paths are combining favorably.
In the case of a mobile receiver, however, the antenna is constantly moving in and out of areas with constructive and destructive interference. This fluctuation of electric field strength degrades the reception and distorts the information carried. If the received information is in the audio range the resulting sound is very offensive to the ear.
Similarly, even in the case where the antenna is in a fixed location, and the location has been selected to be a low noise location, changing multipath induced noise often interrupts the normal noise free reception due to the temporary introduction of a reflective surface, such as low flying airplanes, passing trucks, trains or the like.
In an effort to reduce noise in signals, it has been proposed to use "Auto-correlation" and "Cross-correlation" techniques for noise removal if either the signal or the noise is repetitive. However since most signals do not contain repetitive information this approach is of rather limited practical utility.
An alternate approach proposes to use signal averaging to reduce the signal to ratio noise. However, the enhancement of signal to noise ratio using signal averaging is only proportional to the square root of the number of independent channels that contain the same signal information and tends to increase "signal glitch" as each of the receiving antennas passes through an area of depressed signal.
There is still need for an effective way to reduce noise in analog information in a simple and inexpensive manner especially when the information is not of a repetitive or predictable nature.